Structural panels, also referred to as sandwich panels, comprising a lightweight core material surrounded on both sides by face sheets, find use in a wide variety of applications. Typically they are used where stiffness and structural strength of the panel are primary considerations.
In particular, sandwich panels find use as structural components of aerospace vehicles where the weight of the panel is an equally important consideration. For this reason, so-called honeycomb cores are commonly used in aerospace applications, wherein the core involves sheets of material extending between the face sheets and the majority of the core being open space.
However, further weight reductions are of continuing desirability, and importantly, this must be achieved without sacrificing structural integrity of the panel.
For example, face sheets made of prepreg material are commonly employed. Prepregs comprise at least one sheet of structural fibres, which may be woven or non-woven, which are pre-impregnated with curable resin. Such materials provide lightweight yet strong structural components and so make ideal face sheets for aerospace use.
It is conventional for such face sheets to be adhered to the core material by use of an adhesive. This is to prevent the peeling away of the face sheet from the core material.
This peeling failure mechanism is particularly relevant to honeycomb cores, as the contact area between the face sheet and core can be very small and thus represents a likely primary failure mechanism when such cores are employed.
In an attempt to further reduce the weight of such sandwich panels, so-called self-adhesive prepregs have been suggested which do not require an adhesive, thus providing a weight saving. Examples of self-adhesive prepregs for use on sandwich panels can be found in U.S. Pat. No. 6,045,898, U.S. Pat. No. 6,508,910 and U.S. Pat. No. 6,440,257.
However, by removing the adhesive it has been found that the face panels can be removed more easily requiring a lower peel force. Thus, the reduction in weight may incur a reduction in the structural integrity of such panels.
United States Statutory Invention Registration H000,047 discloses a honeycomb sandwich panel disclosing the use of a spunlaced aramid fabric between a honeycomb core and phenolic resin prepreg face sheets. Although improved peel results are claimed, the peel force is significantly less than would be acceptable for use in a structural application. Additionally, phenolic resins are not suitable for use in a structural aerospace application. Thus, additional weight is introduced without providing an acceptable peel strength.
It would therefore seem that the art has arrived at a point when further reductions in weight can only be achieved by a reduction in structural integrity.